Classifica,on of Solu,ons

Transcription

1 SOLUTIONS Solu,on Homogeneous mixture in which one substance is dissolved in another SOLUTE: substance that is dissolved SOLVENT: substance doing the dissolving INSOLUBLE: does NOT dissolve SOLUBLE: does dissolve Classifica,on of Solu,ons What factors influence solubility? Classifica,on of Solu,ons 1)SATURATED: contains maximum quantity of solute that dissolves at that temperature 2)UNSATURATED: contains less than the maximum amount of solute that can be dissolved 3)SUPERSATURATED: contain more solute than is possible to be dissolved by warming or evaporating (unstable and temporary) Crystalliza,on Opposite of solution formation Greater concentration solute particles greater crystallization More solute particles bumping into each other and become solid Equilibrium established Aqueous Solu,on Solution in which H2O is the solvent Ionic and polar molecules dissolved best WHY? MISCIBLE: liquids completely mix (alcohol and water) IMMISCIBLE: don t mix (oil and water) 1

2 Two Types: Aqueous Solu,on Solution in which H 2 O is the solvent ELECTROLYTES: forms ions, conducts electric current (ex: NaCl or MgCl 2 ) / IONIC NONELECTROLYTES: does not form ions or conduct electric current (ex: Sugar or Ethanol) / POLAR Aqueous Solu,on Solution in which H 2 O is the solvent DISSOCIATION: ionic solid dissolves in water and separates into its ions (+ and -) SOLVATION: process that stabilizes ions and prevents cations and anions from recombining (interactions between the solute and solvent) solute becomes HYDRATED (when solvent is water) IMFs Solu,on Process Interactions in a solution include: solute-solute, solvent-solvent, and solute-solvent Solutions will form when the attractive forces between solute and solvent particles are GREATER than those between the solute-solute or solventsolvent particles alone IMFs Solu,on Process Ion-Dipole IMFs dominate in water / ionic solutions LDFs dominate in nonpolar solute / nonpolar solvent solutions Example: NaCl dissolves easily in water since the IMF s between the Na + and Cl - ions and water are greater (solute-solvent) than the attraction between the ions in the NaCl compound (solute-solute) or between the water s hydrogen bonds (solvent-solvent) Solu,on Forma,on and IMFs In order to dissolve a substance in a solvent: 1) Must overcome (requires energy) Solute-solute IMFs Solvent-solvent IMFs 2) Form solute-solvent attractive forces upon mixing (releases energy) DO NOT SAY LIKE DISSOLVES LIKE ON THE AP EXAM!! Solu,on Forma,on and IMFs POLAR / IONIC dissolve in POLAR: ΔH required to overcome IMFs in both the polar/ionic solute and the polar water molecules is quite LARGE ΔH released due to the interactions between the polar/ionic solute and the polar water molecules is very LARGE Solute can dissolve because it gets as much energy back from the interactions as was required to overcome the IMFs Same goes for nonpolar/nonpolar solutions 2

3 Solu,on Forma,on and IMFs POLAR / IONIC do NOT dissolve in NONPOLAR: ΔH released due to interactions between the polar/ionic solute and the nonpolar solvent is quite SMALL Solute CANNOT dissolve because the energy required to overcome the IMFs is not provided by the solute-solvent interactions Solu,ons and Energy Change Steps in the Solvation Process: 1) Separation of solute into smaller components (expanding the solute) = H 1 2) Overcoming IMFs between solvent molecules (expanding the solvent) = H 2 3) Formation of solute-solvent IMFs = H 3 H soln = H 1 + H 2 + H 3 + H soln = ENDOthermic - H soln = EXOthermic Solu,ons and Energy Change H 1 > 0 ENDOthermic energy input to separate the solute-solute IMFs (HIGH- ionic & polar / LOW- nonpolar) H 2 > 0 ENDOthermic energy input to break the solvent-solvent IMFs (HIGH- polar / LOW- nonpolar) H 3 < 0 EXOthermic energy released is from the attractive forces between the solute-solvent IMFs (HIGH- polar solute and polar solvent / LOW- others) EXAMPLES: Hot Pack- MgSO 4 H soln = kj/mol Solu,ons Cold Pack- NH 4 NO 3 H soln = kj/mol If the H soln is too highly endothermic the solution will NOT form Solu,ons and Energy Change Solu,ons and Energy Change Greater OUT than IN EXOTHERMIC! Greater IN than OUT ENDOTHERMIC! How do solutions form when the heat of solution is positive (endothermic)?... 3

4 Solu,ons and Entropy Solu,ons and Entropy Spontaneous solutions WILL occur if 1) Energy of solution formation is lowered (exothermic) 2) Some are endothermic but still dissolve due to the spreading out of the components into larger volumes resulting in increased ENTROPY (disorder) Solubility Product Constant (K sp ) Saturated solutions have solution in contact with undissolved solute K sp : -Indicates how soluble the solid is in water -ONLY aqueous species -UNITLESS -Coefficients become exponents in expression Ex: BaSO 4 (s) <--> Ba 2+ (aq) + SO 4 2- (aq) K sp = [Ba 2+ ][SO 4 2- ] Solubility and K sp Solubility and K sp are different K sp à equilibrium constant between an ionic solid and its saturated solution -K sp is unitless -As K sp increases so does the amount of solid that dissolves to form a saturated solution Solubility à measured in g/l or moles/l -Amount that dissolves to form a saturated solution -Solubility is affected by ph, other ions in solution (common ions), and temperature SMALLER the K sp = LESS solid will dissolve Solubility Product Constant (K sp ) Solubility Product Constant (K sp ) Calculate the K sp for Mg 3 (PO 4 ) 2 given that its molar solubility is 3.57 x 10-6 M x *NOTE: 3:1 ratio between calcium ion and solubility of calcium phosphate and 2:1 ratio between phosphate ion and calcium phosphate this affects problems! [Ca 2+] must be 3 times solubility of the compound [PO 4 3- ] must be 2 times solubility of the compound Calculate the solubility of CaF 2 and the concentration of each ion (K sp = 4.9 x ). [CaF 2 ] = 2.3 x 10-4 M [Ca 2+ ] = 2.3 x 10-4 M [F - ] = 4.6 x 10-4 M 4

5 Factors Affec,ng Solubility STRUCTURAL EFFECTS: Polar (hydrophilic) dissolve in polar more sites of hydrogen bonding = more soluble (ex: water soluble vitamins) Nonpolar (hydrophobic) dissolve in nonpolar As chains of hydrocarbons become longer, some molecules become less soluble in water and more soluble in nonpolar H H C H O H methanol very soluble in water H H H H H C C C C O H H H H H butanol slightly soluble in water H H H H H H H H H C C C C C C C C O H H H H H H H H H octanol very insoluble in water Factors Affec,ng Solubility STRUCTURAL EFFECTS: Factors Affec,ng Solubility PRESSURE: Little effect on liquids or solids Solubility of a gas INCREASES with INCREASING pressure Example: Sodas are bottled under high pressures Henry s Law Amount of gas dissolved is directly proportional to the pressure of the gas above the solution S = kp S = Concentration (usually M) of dissolved gas k = Constant for solution (mol/l atm) P = partial pressure of gaseous solute above solution Works best for dilute solutions of gases that don t dissociate or react with solvent! Henry s Law A soft drink is bottled so that a bottle at 25 C contains CO 2 gas at a pressure of 5.0 atm over the liquid. Assuming that the partial pressure of CO 2 in the atmosphere is 4.0 x 10-4 atm, calculate the equilibrium concentrations of CO 2 in the soda both before and after the bottle is opened. The Henry s Law constant for CO 2 in aqueous solution is mol/l atm at 25 C. Before: 0.16 M After: 1.2 x 10-5 M Henry s Law 5

6 Factors Affec,ng Solubility TEMPERATURE: SOLIDS INCREASE in temp causes it to dissolve more rapidly and usually causes an INCREASE in solubility GASES Solubility of gases DECREASES with INCREASING temp Factors Affec,ng Solubility TEMPERATURE: Thermal pollution is bad higher water temps mean less oxygen dissolved for wildlife! Concentra,on of Solu,ons Molarity (M) Moles of solute per liter of solution M = Moles of solute Liters of solution Reactions take place when two solutions are mixed In order to do stoichiometric calculations, the CONCENTRATION (amount of solute in a solution) must be known A few ways to express concentration Calculate the molarity of a solution when 11.5 g of NaOH is dissolved in enough water to make 1.5 liters of solution g NaOH 1.5 L soln X 1 mole NaOH 40 g NaOH = 0.19 M PRACTICE: Molarity (M) Moles of solute per liter of solution How many liters of 4.0 M solution can be made using 125 grams of LiBr? 0.36 L What mass of oxalic acid, H 2 C 2 O 4, is needed to make ml of a M solution? 1.12 g Molality (m) Moles of solute per kilogram of solvent m = Moles of solute Kilogram of solvent In lab, 1.0 mole of C 2 H 6 O 2 is dissolved in g of water. Calculate the molality. What is the concentration of a solution that has a volume of 2.5 L and contains 660 g of Ca 3 (PO 4 ) 2? 0.85 M 1.0 mole C 2 H 6 O g H 2 O X 1000 g H 2 O 1 kg H 2 O = 4.0 m 6

7 PRACTICE: Molality (m) Moles of solute per kilogram of solvent How many grams of CaCl 2 are needed to make a 7.9 m solution? 870 g How many grams of H 2 O are required to dissolve 173 g KNO 3 to make a 2.25 m solution? 761 g Mole Frac,on # of Moles of one over the total moles x a = n a n a + n b + A solution consists of 1.0 g of NaCl mixed with 99.0 g of H 2 O. What is the mole fraction of the solute? % by Mass = or Volume Mass or Volume % g or ml of solute g or ml of solution x 100 ppm = Parts Per Million (ppm) Grams of solute Grams of solution x 10 6 What is the percent concentration of 70.0 g of NaCl dissolved in g of water? A 2.5 g sample of groundwater was found to contain 5.4 x 10-6 g of Zn 2+. What is the concentration of Zn 2+ in parts per million? 70.0 g NaCl X 100 = 16.7% ( ) g soln 5.4 x 10-6 g Zn g soln X 10 6 = 2.2 ppm Parts Per Billion (ppb) Preparing Solu,ons ppb = Grams of solute Grams of solution x 10 9 A chemical analysis shows that a water sample contains 8.8 mg of Cd 2+ in a 4.00 x 10 4 g sample. What is the concentration in parts per billion? g Cd x 10 4 g soln X 10 9 = 220 ppb To make a certain concentration of solution, the SOLID should be weighed out first and then placed in a volumetric flask Dissolve the solid in SOME of the solvent then add the remaining solvent 7

8 Preparing Solu,ons To save space, time, and money PREPARED / CONCENTRATED solutions are often purchased Water is then added to DILUTE the stock solutions to the desired concentration Moles of solute BEFORE = Moles of solute AFTER (no solute is added during a dilution) Dilu,ons M 1 V 1 = M 2 V 2 What volume of 16 M H 2 SO 4 must be used to prepare 1.5 L of 0.10 M solution? (16 M) (V 1 ) = (0.10 M) (1.5 L) How much water needs to be added? L or 9.4 ml Add ml of water to make 0.10M! PRACTICE: Dilu,ons M 1 V 1 = M 2 V 2 How many ml of stock solution should be used to prepare ml of M NaBr solution using 2.00 M stock solution? 112 ml Solu,on Stoichiometry Steps for Stoichiometry with Solution Reactions: 1) Write and balance the equation 2) Determine solubility 3) Determine the moles of each reactant and find the limiting reagent 4) Find amount of product Depending on info given, some steps can be skipped!! EXAMPLES: Solu,on Stoichiometry How many grams of Ca(OH) 2 are required to react with 350 ml of 0.40 M HCl? Show the balanced equation. 5.2 g Calculate the mass of solid formed when 1.25 L of M lead (II) nitrate and 2.0 L of M sodium sulfate are mixed. 15 g EXAMPLES: Solu,on Stoichiometry If 625 ml of M aluminum chloride is mixed with 245 ml of M sodium sulfide, how many grams of the precipitate will form AND what is the concentration of EACH ion of the excess reagent that remains in solution after the reaction? 11.6 g [Al 3+ ] = M [Cl - ] = M 8

9 Colliga,ve Proper,es Depend on the total concentration of solute particles present NOT the type IMFs of the solvent are interrupted when the solute is added changing the properties of the solvent Adding solute to the solvent changes the following physical properties: vapor pressure lowering, boiling-point elevation, freezing-point depression, and osmotic pressure Vapor Pressure Lowering Presence of a nonvolatile solute LOWERS the VP of a solvent Volatile (has a VP) / Nonvolatile (no VP) Solute gets in the way preventing fewer solvent molecules on the surface from escaping COOKING PASTA (Add salt to the water higher altitudes would take LONGER to cook because boil at lower temp) Vapor Pressure Lowering Vapor Pressure Lowering PURE SOLVENT SOLUTE / SOLVENT Vapor Pressure Lowering Raoult s Law IDEAL solutions P A = X A P A (like ideal gases) obey this law! P A = Partial pressure exerted by the solvent s vapor above the solution X A = mole fraction of the solvent P A = vapor pressure of the pure solvent 9

10 Raoult s Law Glycerin (C 3 H 8 O 3 ) is a nonvolatile nonelectrolyte with a density of 1.26 g/ml at 25 C. Calculate the VP at 25 C of a solution made by adding 50.0 ml of glycerin to ml of water. The VP of pure water at 25 C is 23.8 torr, and its density is 1.00 g/ml torr (about 0.6 torr LOWER) Boiling-Point Eleva,on Presence of a solute ELEVATES the boiling point Nonvolatile solute causes the VP to decrease more energy must be added to get VP equal to atmospheric Solute gets in the way of the solvent molecules and impede their escape from the surface ANTIFREEZE (Operate engine at higher temps) COOKING PASTA (Add salt to the water) Boiling-Point Eleva,on Boiling-Point Eleva,on Boiling-Point Eleva,on ΔT bp = K b m i ΔT bp = increase in boiling point (add to normal boiling point for final answer) K b = proportionality constant that depends on the SOLVENT m = molality i = Van t Hoff factor Van t Hoff Factor Colligative properties depend on number of particles soluble ionic compounds dissolve to give 2 or more moles of particles (NaCl à Na + + Cl - ) Molecular compounds do not dissociate and form only 1 mole of particles Ex: CaCl 2 i = 3 / (NH 4 ) 3 PO 4 i = 4 In absence of any actual info about i, just use the ideal value (i = 1) organic compounds i = 1 10

11 Boiling-Point Eleva,on At what temperature does a solution of 58.5 g of salt dissolved in g of water boil? (K b = C/m; i=2) 1.02 C raised 101 C boils Freezing-Point Depression Presence of a solute LOWERS the freezing point Nonvolatile solute causes the VP to decrease less energy added to get VP of solid and liquid to be same Solute gets in the way of the solvent molecules and prevent them from clustering together ANTIFREEZE (Ethylene Glycol, Alcohols, etc.) SALTING ROADS (NaCl, CaCl 2, etc.) Freezing-Point Depression Freezing-Point Depression SURVIVING THE WINTER! Freezing-Point Depression ΔT fp = K f m i ΔT fp = decrease in freezing point (subtract from normal freezing point for final answer) K f = proportionality constant that depends on the SOLVENT m = molality i = Van t Hoff factor Freezing-Point Depression What is the molar mass of nicotine if g dissolved in g of water causes it to freeze at C. (K f = 1.86 C/m) 163 g/mol 11

12 Osmo,c Pressure Pressure that STOPS osmosis OSMOSIS movement of a solvent through selectively permeable membrane toward HIGHER solute conc. Osmo,c Pressure π = M R T i π = osmotic pressure in atm M = molarity of the solution OR M = n/v R = L atm / K mol i = Van t Hoff factor Usually used to find MM of unknown better than FPD or BPE! Osmo,c Pressure Phase Diagram To determine the molar mass of a certain protein, 1.00 x 10-3 g of it was dissolved in enough water to make 1.00 ml of solution. The osmotic pressure of this solution was found to be 1.12 torr at 25.0 C. Calculate the molar mass of the protein x 10 4 g/mol Colloids Solutions with particles that are too big to dissolve but too small to settle out Colloidal dispersions are the dividing line between solutions and heterogeneous mixtures Tiny particles suspended in some medium TYNDALL EFFECT: scattering of light particles light passes through a solution but is scattered by a colloid Colloids Solutions with particles that are too big to dissolve but too small to settle out 12

13 Solubility Rules Knowing whether substances are soluble or insoluble tells us if a precipitate forms 1) Salts of ammonium and alkali metals are always soluble 2) All chlorides, bromides, and iodides are soluble except when combined with Ag, Hg 2+, and Pb which are insoluble 3) Chlorates, acetates, and nitrates are soluble 4) Sulfates are soluble except with Ca, Sr, Ba, Hg, Pb, and Ag which are insoluble Solubility Rules Knowing whether substances are soluble or insoluble tells us if a precipitate forms 5) Phosphates, carbonates, and sulfides are insoluble except ammonium and alkali metal compounds are soluble 6) All metallic oxides are insoluble except ammonium and alkali metal compounds are soluble 7) All hydroxides are insoluble except ammonium, alkali metal compounds, and group 2A from Ca down are soluble Ex: Mg(NO 3 ) 2 (aq) + 2 NaOH (aq) è Mg(OH) 2 (s) + 2 NaNO 3 (aq) Solubility Rules PRACTICE: Classify each of the following as soluble or insoluble. Sodium carbonate Lead sulfate Barium nitrate Ammonium phosphate Cobalt (II) hydroxide 13